The present invention relates to a semiconductor laser control device for use in a photo (photomagnetic) disc, and more particularly, to a semiconductor laser control device comprising a current supplying source for signal reproduction and a current supplying source for recording and erasing and controlling beam output of the semiconductor laser.
FIG. 3 shows a conventional semiconductor laser control device. In the constitution shown in the figure, a controller controls all portions generally to control finally the output of semiconductor laser 6.
When reading (reproducing) signals recorded on an optical (photomagnetic) recording medium, first current supplying source 2 generates a current for signal reproduction, and semiconductor laser 6 receives the current to radiate. A part of the beam output of the semiconductor laser is subjected to photoelectric conversion conducted by beam-detecting monitor 7 provided integrally with or in the vicinity of the semiconductor laser 6, thus the beam output is obtained through monitor output processing element 8. Further, the beam output thus obtained is compared by processing circuit 9 with a reference value from controller 1, and the result of the comparison is subjected to sample-and-hold by means of sample-and-hold circuit 10. The result of the sample-and-hold causes the value of current from the second current supplying source 2 to be subjected to feedback control. In the manner mentioned above, the beam output for reproduction is controlled on a real time basis during reproduction so that the beam output may show the predetermined value.
On the other hand, when recording or erasing, second current supplying source 3 generates current for signal recording or signal erasing, and the current for recording and the current for reproduction are added in current adder 5 and the added current is received by the semiconductor laser 6 which radiates. A part of the beam output of the semiconductor laser is subjected to photoelectric conversion conducted by beam-detecting monitor 7 provided integrally with or in the vicinity of the semiconductor laser 6, thus the beam output is obtained through monitor output processing element 8. Further, the peak value held by peak-hold circuit 11 is compared by processing circuit 12 with a reference value from the controller 1. The result of the comparison causes the current value of the second current supplying source 3 to be subjected to feedback control. In the manner mentioned above, the beam output for recording or erasing is controlled on a real time basis so that the beam output may show the predetermined value.
In the constitution mentioned above, it is required that the controlling system operates at high speed because the beam output is controlled during operation.
A beam output for recording, in particular, is composed of pulses having an extremely narrow width. FIG. 4 shows that condition. Pulses shown in FIG. 4 (a) have a cycle period of 167 ns (6 MHz), for example, and pulse width thereof is 30 ns. Pulse rise time is about 5 ns.
Since each pulse is different from others during the operation of recording, each pulse is required to be detected in high fidelity. Therefore, beam-detecting monitor 7 and monitor output processing element 8 are required to be capable of operating at high speed for detecting the pulses in high fidelity. Since the pulse has an extremely short cycle, it is necessary to provide peak-and-hold circuit 11 in the previous step of processing circuit 12 to obtain what is shown in FIG. 4 (b).
A method of averaging wave height values instead of detecting each pulse is also considered, but it is feared that accuracy may be lowered.
The invention has been achieved to solve the problems mentioned above, and its object is to realize a semiconductor laser control device capable of being composed of simple circuits using no high speed circuit elements.